A characterization of the data, including its intended use and limitations.

Abstract:

Pursuant to guidance provided in the Department of Environmental Quality (DEQ) policy memorandum entitled "Policy for Addressing Degraded Ground Water Quality Areas" dated March 1, 2000, a statewide list of significantly degraded areas with nitrate was developed. In 2002, the DEQ, in cooperation with the Idaho Ground Water Monitoring Technical Committee (GWMTC) published a ranking of 25 Nitrate Priority Areas (NPAs). The NPAs are areas where elevated levels of nitrate have been found in ground water. In 2008, incorporating recent monitoring, 32 NPA's were evaluated and prioritized.
DEQ, in conjunction with the GWMTC revised the NPAs published in 2002. The revisions utilized data collected since the original NPAs were developed to evaluate ground water quality changes in existing NPAs and to identify new areas with nitrate degraded ground water. In the summer of 2006 the GWMTC began the process of revising the NPAs. Based on experience gained since 2002, which provided agencies with a better understanding of the resources necessary to address ground water degradation over large areas, it was decided to limit the effort to delineating only Priority I NPAs and not include Priority 2 NPAs.
DEQ, in conjunction with the GWMTC revised the NPAs published in 2002. The revisions utilized data collected since the original NPAs were developed to evaluate ground water quality changes in existing NPAs and to identify new areas with nitrate degraded ground water. In the summer of 2006 the GWMTC began the process of revising the NPAs. Based on experience gained since 2002, which provided agencies with a better understanding of the resources necessary to address ground water degradation over large areas, it was decided to limit the effort to delineating only Priority I NPAs and not include Priority 2 NPAs.
The minimum criterion for a Priority 1 NPA is 25% of sampled wells have nitrate levels at or above 5 milligrams per liter (mg/L). The state and federal drinking water standard, as well as the Idaho Ground Water Quality Standard for nitrate is 10 mg/L. Within a Priority 2 NPA 50% of the sampled wells have nitrate levels at or above 2 mg/L. Almost all developed areas of the state meet this criterion.

Purpose:

The NPA Ranking Process (Ranking Process), developed by DEQ, in consultation with the GWMTC, provides the rationale for numerically ranking areas in Idaho with identified ground water degradation from nitrates. The statewide priority list created through this process will be used to prioritize the implementation of protective management strategies or corrective action measures within the NPAs.

Supplemental information:

In the fall of 2006, DEQ began collecting and compiling nitrate results and well location data from the numerous agencies monitoring ground water quality in Idaho. Well location information, sampling date, and nitrate concentration data were received and compiled by DEQ. Spatial information was reconciled and integrated into a Geographic Information System (GIS) coverage.
Sources of data included the DEQ public water system database, the Statewide Ambient Ground Water Quality Monitoring Network (Statewide Program), numerous U.S. Geological Survey (USGS) studies, DEQ regional and local monitoring projects, regional studies conducted by the Idaho State Department of Agriculture (ISDA), and dairy sampling by ISDA. Data from 1990 into 2007 were plotted with the corresponding nitrate value. The 2002 NPAs were based on data from 1974 to 2000. The use of more recent data resulted in a difference of sample numbers between the 2002 and 2008 NPA analysis within a few areas with roughly the same configuration, namely Fort Hall and Bruneau. For sites with multiple values the most recent value was used. Data from site-specific monitoring projects associated with known point sources of nitrate contamination were not included in the data set.
Ground water quality data from about 9,950 wells statewide were compiled and evaluated to develop the 2008 NPAs. The NPAs contain 3,600 of the 9,950 wells and encompass a combined area of 2,229,048 acres. Approximately 300,000 people are estimated to live within the boundaries of the NPAs.
Phase 2 - Delineation of Nitrate Priority Areas
Once the data was located spatially with the corresponding nitrate concentration, the NPAs were delineated. The GWMTC reviewed a variety of approaches and discussed pros and cons of different methods to delineate NPAs. A single method, which would provide concise, objective, scientifically defensible boundaries, was the ultimate goal of the GWMTC. The GWMTC, after much discussion and multiple attempts to use a single method, determined that a triad of different methods provided the most objective NPA delineation. To decrease the reliance on the potential subjectivity of professional judgment, two geostatistical methods, indicator kriging and ordinary kriging, were incorporated in the process. Geostatistical software packages for indicator kriging and ordinary kriging, available for ESRI® ArcMapTM, were applied to the data. The two geostatistical techniques and professional judgment factors are described in very simplified terms below.
" Indicator kriging considers if a value is above or below a specific concentration. It analyzes the data and shows the probability of exceeding a specific concentration. The method allows the user to use any combination of probability and concentration. For this process a 25% probability of exceeding 5 mg/L for nitrate was used.
" Ordinary kriging interpolates values between locations with data and contours the data. Areas located within the contour interval of 3.5 mg/L were used.
" The third component of the process included professional judgement. This component included the consideration of land use and knowledge of aquifers and hydrogeologic factors. For example, efforts were made to not extend NPA boundaries into undeveloped lands.
The utilization of geostatistical methods provided more objective and scientifically defensible boundaries. It should be noted that the NPA boundaries are considered estimations that identify general areas where nitrate levels are more likely to be elevated. NPAs may be considered analogous to climate zones or precipitation maps, which constantly change. Distinct NPA boundaries may not be appropriate because of the dynamic nature of ground water systems. Nitrate levels may fluctuate seasonally or annually for a number of reasons including flow direction or water level changes in response to irrigation practices or seasonal land use practices. Additionally, because nonpoint sources of contamination do not have distinct contamination plumes like point source releases, the boundaries of NPAs are not definite.

Dataset credit:

IDEQ

Language of dataset: en

Point Of Contact

Contact information for the individual or organization that is knowledgeable about the data.

Organization: Idaho Department of Environmental Quality

Person: Tonia Mitchell

Position: Sr. Ground Water Hydrogeologist

Phone: 1-208-373-0502

Fax: 1-208-373-0143

Email: Tonia.Mitchell@deq.idaho.gov

Hours of service: 8:00 - 5:00 PM

Address type: mailing and physical address

Address:

1410 North Hilton

City: Boise

State or Province: Idaho

Postal code: 83706-1255

County: USA

Data Type

How the data are represented, formatted and maintained by the data producing organization.

Restrictions and legal prerequisites for accessing or using the data after access is granted.

Access constraints:

None

Use constraints:

Restriction of Liability: Neither the State of Idaho nor the Idaho Department of Environmental Quality, nor any of their employees make any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, completeness or usefulness of any information or data provided. Metadata is provided for all data sets, and no data should be used without first reading and understanding its limitations. The data could include technical inaccuracies or typographical errors. The Department of Environmental Quality may update, modify, or revise the data used at any time, without notice. The distributor shall not be held liable for improper or incorrect use of this data, based on the description of appropriate/inappropriate uses described in this metadata document.
The boundaries of the Nitrate Priority Areas are not considered stationary, are subject to change upon receipt of additional water quality data, and are not intended to be used as regulatory designations. Nitrate Priority Areas are intended for agencies to use for prioritizing and focusing resources to develop and implement ground water quality improvement strategies with local communities, per DEQ Policy Memorandum PM-004.

Data Security Information

Handling restrictions imposed on the data because of national security, privacy or other concerns.

Security classification: Restricted

Cross Reference

Information about other, related data sets that are likely to be of interest.

Information about events, parameters, tolerances and techniques applied to construct or derive the data.

Process step information

Process Step 1

Process description:

In the fall of 2006, DEQ began collecting and compiling nitrate results and well location data from the numerous agencies monitoring ground water quality in Idaho. Well location information, sampling date, and nitrate concentration data were received and compiled by DEQ. Spatial information was reconciled and integrated into a Geographic Information System (GIS) coverage.
Sources of data included the DEQ public water system database, the Statewide Ambient Ground Water Quality Monitoring Network (Statewide Program), numerous U.S. Geological Survey (USGS) studies, DEQ regional and local monitoring projects, regional studies conducted by the Idaho State Department of Agriculture (ISDA), and dairy sampling by ISDA. Data from 1990 into 2007 were plotted with the corresponding nitrate value. The 2002 NPAs were based on data from 1974 to 2000. The use of more recent data resulted in a difference of sample numbers between the 2002 and 2008 NPA analysis within a few areas with roughly the same configuration, namely Fort Hall and Bruneau. For sites with multiple values the most recent value was used. Data from site-specific monitoring projects associated with known point sources of nitrate contamination were not included in the data set.
Ground water quality data from about 9,950 wells statewide were compiled and evaluated to develop the 2008 NPAs. The NPAs contain 3,600 of the 9,950 wells and encompass a combined area of 2,229,048 acres. Approximately 300,000 people are estimated to live within the boundaries of the NPAs.
Phase 2 - Delineation of Nitrate Priority Areas
Once the data was located spatially with the corresponding nitrate concentration, the NPAs were delineated. The GWMTC reviewed a variety of approaches and discussed pros and cons of different methods to delineate NPAs. A single method, which would provide concise, objective, scientifically defensible boundaries, was the ultimate goal of the GWMTC. The GWMTC, after much discussion and multiple attempts to use a single method, determined that a triad of different methods provided the most objective NPA delineation. To decrease the reliance on the potential subjectivity of professional judgment, two geostatistical methods, indicator kriging and ordinary kriging, were incorporated in the process. Geostatistical software packages for indicator kriging and ordinary kriging, available for ESRI® ArcMapTM, were applied to the data. The two geostatistical techniques and professional judgment factors are described in very simplified terms below.
" Indicator kriging considers if a value is above or below a specific concentration. It analyzes the data and shows the probability of exceeding a specific concentration. The method allows the user to use any combination of probability and concentration. For this process a 25% probability of exceeding 5 mg/L for nitrate was used.
" Ordinary kriging interpolates values between locations with data and contours the data. Areas located within the contour interval of 3.5 mg/L were used.
" The third component of the process included professional judgement. This component included the consideration of land use and knowledge of aquifers and hydrogeologic factors. For example, efforts were made to not extend NPA boundaries into undeveloped lands.
The utilization of geostatistical methods provided more objective and scientifically defensible boundaries. It should be noted that the NPA boundaries are considered estimations that identify general areas where nitrate levels are more likely to be elevated. NPAs may be considered analogous to climate zones or precipitation maps, which constantly change. Distinct NPA boundaries may not be appropriate because of the dynamic nature of ground water systems. Nitrate levels may fluctuate seasonally or annually for a number of reasons including flow direction or water level changes in response to irrigation practices or seasonal land use practices. Additionally, because nonpoint sources of contamination do not have distinct contamination plumes like point source releases, the boundaries of NPAs are not definite.
The delineation process yielded 32 Priority 1 NPAs. These NPAs are located throughout Idaho: stretching from Nez Perce County in northcentral Idaho to Franklin County in southeast Idaho and from Owyhee County in southwest Idaho to Fremont County in eastern Idaho. The spatial distribution of NPAs is very similar to the distribution in 2002. Figure 1 is a map showing a comparison between the 2002 and 2008 NPAs. Some of the increase in the number of NPAs is the result of splitting some of the larger NPAs in the Treasure Valley into smaller NPAs. For example the 2002 Homedale-Marsing NPA was separated into two NPAs: one Homedale and one Marsing. The separation was based on data showing an area with lower nitrate concentration located between the two NPAs.
New NPAs, not divided from larger areas, that were identified by more recent data include: the Lindsay Creek area near Lewiston; an area in Gem County; areas west of Middleton; Mountain Home Air Force Base; Hagerman; an area east of Blackfoot, and the Mink Creek drainage to the south of Pocatello.
Based on more recent sampling, two of the 2002 lower ranking NPAs were removed from the NPA list because the results did not meet the criterion of 25% of sampled wells at or above 5 mg/L. These areas include Genesee/Cow Creek located to the north of Lewiston, and Hibbard near Rexburg.
Phase 3 - Nitrate Priority Area Ranking
The process used to rank the NPAs in 2002 was also used to rank the 2008 NPAs. The GWMTC supported the continued use of the ranking process to maintain consistency with previous efforts. Additionally, the ranking process used in 2002 went through a 60-day public comment and was revised based on comments received during that period.
The NPA Ranking Process (Ranking Process), developed by DEQ, in consultation with the GWMTC, provides the rationale for numerically ranking areas in Idaho with identified ground water degradation from nitrates. The statewide priority list created through this process will be used to prioritize the implementation of protective management strategies or corrective action measures within the NPAs.
The ranking process employed an approach intended to:
" minimize subjectivity,
" have statewide applicability,
" be transferable to other types of contaminants, such as pesticides, and
" use existing information.
The ranking process considers three weighted principal criteria: population, existing water quality, and water quality trends. A secondary criterion, impacts to beneficial uses other than potable water supply, is considered to a lesser extent because it is not directly related to public health. The secondary criterion is included to comply with the DEQ Policy Memorandum entitled "Policy for Addressing Degraded Ground Water Quality Areas."
Following a 30-day public comment period for the 2008 ranking, the GWMTC reviewed the public comments, and based on the comments, decided to adjust point allocations in scoring the population category. The point allocations were revised with more refined categories for the number of wells at or above the maximum contaminant level, the number of source water protection areas or public water systems and the population within the degraded area.
Criteria and Scoring Format
The criteria and scoring format are described below.
Primary Criterion
1) Population - The population criterion considers the number of people living in an area that are potentially drinking nitrate-degraded water. This criterion consists of an assessment and point assignment of three elements.
a. Population within the priority area. This element is based upon census data. From 1 to 3 points may be accrued at this stage. One point is assigned to areas with populations less than 5,000; 2 points are assigned to areas with populations between 5,000 and 50,000; and 3 points are assigned to areas with populations of 50,000 or greater. Example - Population =5853 is between 5000 to 50,000 and would be assigned 2 points.
b. Source Water Protection Areas or Public Water System Wells within the priority area. The DEQ Policy Memorandum "Addressing Degraded Ground Water Quality Areas" directs DEQ to consider source water assessment areas in ranking the priority areas. Source water protection areas (SWPAs), or "capture zones," represent the aerial extent of 3-, 6-, and 10-year travel times for ground water to reach the Public Water System (PWS) well or SWPA. The score is dependent on the number of source water assessment/protection areas or PWS wells within a NPA.
This stage provides 0, 1, 2, or 3 points. Areas without a PWS well do not receive points. Areas with 1 to 20 PWS wells or SWPAs receive 1 point; areas with 21 to 40 receive 2 points; and areas with more than 40 PWS wells or SWPAs receive 3 points. Example - PWS wells or SWPAs in Priority Area=11 is between 1 and 20 and would be assigned 1 point.
c) Number of Wells with Nitrate Concentrations above 10 mg/L. The GWMTC determined the number of wells with nitrate exceeding 10 mg/L was an important ranking factor. Furthermore, the number of sampled wells with nitrate greater than or equal to 10 mg/L within the priority area is representative of the potential for the public to ingest contaminated ground water. This step is intended to equalize the scoring of large populations drinking water from uncontaminated sources with small populations drinking water from nitrate contaminated sources. Nitrate contamination greater than or equal to 10 mg/L is the only factor tallied.
Points are accumulated as follows: 0 wells = 0 points, 1 to 5 wells = 1 point, 6 to 20 wells = 2 points, 21 to 40 wells = 3 points; and greater than 40 wells = 4 points.
Example - Number of Wells with Nitrate greater than 10 mg/L = 29 wells is assigned 3 points.
At this stage the population scores are subtotaled. Example - (2 + 1 + 3 = 6)
2) Water Quality - This criterion considers the concentration of nitrate contamination with respect to drinking water standards. The criterion is based on the percent of sampled wells with ground water nitrate concentrations greater than or equal to 2 mg/L, 5 mg/L, and 10 mg/L respectively. These categories were selected to maintain consistency with existing data formats used by the GWMTC.
a) Percentage of wells with ground water nitrate concentrations greater than or equal to 2 mg/L. This concentration threshold provides an indication of human-caused (anthropogenic) impacts. The upper limit for naturally occurring (background) concentrations of nitrate is considered to be about 2 mg/L. Points are accumulated by multiplying the percentage of sampled wells by 2. Example - 88% of the wells sampled equaled or exceeded 2 mg/l. (0.88 x 2 = 1.76).
b) Percentage of wells with ground water nitrate concentrations greater than or equal to 5 mg/L. This nitrate concentration is considered evidence of significant degradation. This concentration represents one half the drinking water standard for nitrate of 10 mg/L. Public drinking water systems are required to increase monitoring frequency when this level is reached. Because these wells are a subset of the wells containing nitrate greater than or equal to 2 mg/L, this percentage is always less than or equal to the percentage of wells above 2 mg/L. Points are accumulated by multiplying the percentage of sampled wells by 5. Example -73% of the wells sampled equaled or exceeded 5 mg/l. (0.73 x 5 = 3.65).
c) Percentage of wells with ground water nitrate concentrations greater than or equal to 10 mg/L. State of Idaho and federal drinking water standard maximum contaminant level for nitrate is 10 mg/L. Nitrate concentrations above this level present health risks to certain individuals. Because these wells are a subset of the wells containing nitrate at or above 5 mg/L nitrate concentration, this percentage is always less than or equal to the percentage of wells greater than or equal to 5 mg/L. Example - 45% of the wells sampled equaled or exceeded 10 mg/l. (0.45 x 10 = 4.50).
The sum of all three factors above gives the final water quality score.
Example - (1.76 + 3.65 + 4.50 = 9.91 points)
3) Water Quality Trends - This criterion considers water quality trends within each priority area. Determining water quality for a specific priority area is a complex process requiring a comprehensive analysis of water quality data. The Idaho Department of Water Resource (IDWR) evaluated the nitrate data using statistical methods to determine if statistically significant water quality trends are present in the areas. To minimize the potential for bias from data sets grouped due to intense investigation at nitrate contaminated sites, the GWMTC decided to use an average value from the site in the trend analysis. The concentrations of nitrate are classified as increasing, no discernable trend, or decreasing. Due to data limitations three NPAs (Bruneau, Hagerman, and Notus) are listed with a trend of insufficient data. These three (3) NPAs were assigned to the no discernable trend category. The trend analysis results are summarized in the IDWR Water Information Bulletin Number 50, Part 7 entitled "Trend Analysis for Idaho's Nitrate Priority Areas, 1994 - 2007". The report is available on the IDWR website at the following URL. http://www.idwr.idaho.gov/hydrologic/info/pubs/wib/wib50p7_nitrate_trend_analyses.pdf
This criterion will be assigned a maximum value of 10 points. The scoring breakdown is listed below:
a) Increasing = 10 points
b) Static or no discernable trend = 5 points
c) Decreasing = 0 points
Example - Increasing Nitrate Trend is assigned 10 points.
Secondary Criterion
4) Other Beneficial Uses - The "Other Beneficial Use" criterion is included in the process because DEQ policy states that this is to be a consideration in ranking the NPAs. However, this factor does not appear to be an issue in any of the existing NPAs with the exception of the Twin Falls and Hagerman areas of aquaculture. When other beneficial uses are impacted, one point will be added to the score. Aquaculture is an example of a beneficial use potentially impacted by elevated nitrates.
Example - no other beneficial uses = 0 points
Total Score
The scoring format is to total the scores from the three (3) primary criteria subtotals and from the one (1) secondary criterion. Total scores are the sums of the population, water quality, water quality trend, and other beneficial uses. Total Scores are used to rank the NPAs and are displayed in Table 1.
Example: Total Score - (6 + 9.91 + 10 + 0 = 25.91)
The final score remains to the hundredth decimal to distinguish between numerically close scores.

Description of the data known by the party from whom the data may be obtained, liability of party distributing data, and technical capabilities required to use the data.

Resource description:

2008 Idaho Nitrate Priority Areas in ground water.

Distribution liability:

Neither the State of Idaho nor the Idaho Department of Environmental Quality, nor any of their employees make any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, completeness or usefulness of any information or data provided. Metadata is provided for all data sets, and no data should be used without first reading and understanding its limitations. The data could include technical inaccuracies or typographical errors. The Department of Environmental Quality may update, modify, or revise the data used at any time, without notice. The distributor shall not be held liable for improper or incorrect use of this data, based on the description of appropriate/inappropriate uses described in this metadata document.
The boundaries of the Nitrate Priority Areas are not considered stationary, are subject to change upon receipt of additional water quality data, and are not intended to be used as regulatory designations. Nitrate Priority Areas are intended for agencies to use for prioritizing and focusing resources to develop and implement ground water quality improvement strategies with local communities, per DEQ Policy Memorandum PM-004.

Distribution Point of Contact

Contact information for the individual or organization distributing the data.

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